Partial sequencing of anonymous cDNA clones (Expressed Sequence Tags, ESTs) is a rapid and cost effective method for the generation of data on coding capacity of genomes. In plants, this method was first used for Arabidopsis thaliana and Oryza sativa (Höfte et al. 1993; Cooke et al. 1996; Rounsley et al. 1996; Yamamoto et al. 1997). It has subsequently been used for many other plant species such as Medicago, Mesembryanthemum, wheat, barley, tomato, potato, pine and sunflower (Györgyey et al. 2000; Ozturk et al. 2002; Van der Hoeven et al. 2002; Ronning et al. 2003 Allona et al. 1998). This abundance of sequence information presents opportunities to accelerate progress towards understanding genetic mechanisms that control plant growth and responses to the environment. Environmental factors such as drought, extreme temperatures, high or fluctuating salinity can affect plant growth and performance and in the case of agronomically important plants, this may translate to reduced yield. In particular, increasing soil salinisation in irrigated areas has necessitated the identification of crop traits/genes which confer resistance to salinity (Cushman and Bohnert 2000).
Hyperosmotic stress, such as that caused by exposure of cells to high concentrations of NaCl causes imbalance of cellular ions, change in turgor pressure and cell volume and alters the activity and stability of macromolecules. Characterization and cloning of plant genes that confer salinity tolerance remains a challenge. Genetic studies revealed that tolerance to drought and salinity in some crop varieties is principally due to additive gene effects. Physiological and biochemical responses to high levels of ionic or nonionic solutes and decreased water potential have been studied in a variety of plants. Based on accumulated experimental observations and theoretical consideration, one suggested mechanism that may underlie the adaptation or tolerance of plants to osmotic stresses is the accumulation of compatible, low molecular weight osmolytes such as sugar alcohols, special amino acids and glycinebetaine (Greenway and Munns, 1980). In addition to metabolic changes and accumulation of low molecular weight compounds, a large set of genes is transcriptionally activated which leads to accumulation of new proteins in vegetative tissue of plants under osmotic stress conditions (Skriver and Mundy, 1990).